Disequilibrium phenocrystic assemblage within dacites reveals magma mixing and stratified chamber after crustal assimilation at El Hoyazo volcano, SE Spain

Abstract

In volcanic areas where evolved and primitive magmas coexist, it is common to observe evidence of magma mixing. At the Neogene silicic volcano of El Hoyazo in SE Spain, the detailed petrological and geochemical characteristics of the phenocrysts (plagioclase, biotite, amphibole and pyroxene) as well as their host dacitic lavas allow the reconstruction of the structure of the magma plumbing system. Microtextures, modal abundances, and geochemistry of zoned phenocrysts reveal a stratified magma composed of (i) high-SiO2 dacites with An-poor (<50%) plagioclase and Mg-poor biotite; (ii) high-SiO2 andesites with intermediate-An (50–70%) plagioclase and intermediate-Mg biotite; and (iii) an andesitic magma with An-rich (>80%) plagioclase, Mg-rich pyroxene, and tschermakitic hornblende, whose zoning records different crystallization depths during magma ascent. Evidence from the hornblende varieties and from mafic enclaves or inclusions indicates that the stratified chamber was repeatedly injected by hotter magmas from greater depth.